1. Field of the Invention
The present invention relates to an oscillograph or direct writing chart recorder employing a pressurized fluid pen inking system and a Z fold paper feed and to an improved recording method.
2. Description of the Prior Art
A chart recorder is commonly used to provide a graphic record of transient phenomena. In such a recorder, a strip of paper is moved past a pen at a selected speed. The pen is driven by a motor, typically a D'Arsonval movement. The phenomena to be recorded is converted to a proportional electrical current which is passed through the coil of the D'Arsonval movement to deflect the pen and provide the graphic record on the moving paper strip.
Various types of pens may be used to provide the recording. A simple capillary system may be used. However, the acceleration forces on the pen arising during transient deflection may exceed the capillary force on the ink, causing drops of ink to be whipped off the pen. Starting the flow of ink and preventing drying of the ink in periods of non-use are also a problem.
Systems without ink have been developed. A thermally sensitive composition may be applied to the paper. The pen comprises a heated stylus which, when applied to the thermally sensitive composition, provides visually observable indicia. However, rapid movements of the pen reduce the width of the line, making the graphic record difficult to read.
A pen system which overcomes many of the foregoing disadvantages is the hydrostatic pressure inking system described in U.S. Pat. No. Re. 25,692 to Brown, Jr. In this system the pen comprises a hollow tube. A viscous ink is forced through the tube under pressure and the tip of the pen is sealed against the paper by a bias force correlated to the ink pressure. The system works best with a chart paper having a hard finish formed by a clay coating or filler.
The strip of paper is commonly supplied to the recorder in a roll as shown, for example, in the Brown, Jr. patent. However, such rolls are awkward to store prior to use. In the machine, the entire roll must be accelerated each time the strip is advanced through the machine. The inertia of the roll places a limit on the size of the roll which may be used and the variations in the inertia and torque arm as the roll is consumed make control difficult. The roll causes a curl in the strip which varies as the diameter of the roll varies. The paper record, with its curl, is often hard to store.
Other types of paper feed have been considered, including a Z folded strip of paper. Such a strip is alternately folded in opposite directions at predetermined locations so as to form a "Z". The strip contains transverse lines of perforations at the predetermined locations to effect the folding. The strip, when so folded, lies in a stack.
A Z fold feed avoids many of the problems of a roll fold. The amount of paper which must be accelerated during movement through the recorder is minimal and remains relatively constant. The height of the stack may be usually easily increased to increase the size of the paper supply. Excessive curl is not present and the strip may be separated at the folds to provide sheets which are easily handled and filed.
However, there has not heretofore been available a commercially successful recorder employing a Z fold paper feed and a pressure ink system of the type disclosed in the aforesaid U.S. Patent. This is due to one or more of the following reasons. The pen tended to hang up on the perforations at the folds, particularly since it is forced onto the surface of the paper with the sealing pressure. Ink from the pen bled through at the perforations. The paper fibers exposed at the perforations clogged the pen.
To obtain some of the advantages of a Z fold feed in a pressure ink recorder, it has been proposed to provide such a recorder using a roll feed with paper perforated at predetermined intervals. The roll is perforated from the back side in an effort to maintain the integrity of the coating as complete as possible, thereby to accommodate the pen of the pressure ink system. After the strip passed under the pen, it was Z folded by machine or by hand. Unfortunately, such a recorder tended to have all of the disadvantages of both a roll fed device and a Z fold fed device and few of its advantages.